This invention relates to an electrostatically focusing type image pickup tube and more particularly to a photoconductive electrostatically focusing type image pickup tube.
Since the resolution characteristic of an image pickup tube is influenced greatly by the quality of the electron lens system it is necessary to carefully design the electrode construction of the electron lens system so as to obtain satisfactory operating characteristics and to make easy the method of manufacturing. Generally speaking, in order to obtain a high resolution, it is necessary to focus the electron beam radiated from the cathode electrode of an image pickup tube such that the electron beam will have a cross-sectional configuration of a circle hving a diameter of from 10 to 30 microns, and cause it to impinge upon the target, For this reason, in an electrostatically focusing type image pickup tube, the electrode group comprising the electron lens system is positioned as close as possible to the inner surface of the tube so as to increase as far as possible the diameter of the focusing lens for the purpose of improving the resolution.
FIG. 1 shows a diagrammatic longitudinal sectional view of one example of a prior art electrostatically focusing type image pickup tube having a diameter larger than one inch. The image pickup tube shown in FIG. 1 comprises a cylindrical bulb 1, a target 2 mounted on one end of the bulf and including a face plate, a transparent conductive film, a photoconductive film, and an output terminal (not shown), a stem 4 mounted on the other end of the bulb and provided with a plurality of lead pins 3 and a plurality of electrodes contained in the bulb. The electrodes are arranged coaxially as will be described later in detail. The interior of the bulb 1 is evacuated to a high vacuum. These electrodes comprise a getter 5, a cathode electrode 6, a first grid electrode 7, and a second grid electrode 8. An apertured electrode 8a having a small perforation for passing an electron beam is secured to one side of the second grid electrode which faces target 2, the perforation serving to restrict the electon beam emanated from the cathode electrode 6 for producing a fine beam. The cathode electrode 6 and the first and second grid electrodes 7 and 8 cooperate to constitute a three electrode electron gun structure. There are also provided a cylindrical third grid electrode 9 having a reduced diameter portion on its side facing the cathode electrode 6, a cylindrical fourth grid electrode 10 having reduced diameter portions on both ends, and a cylindrical fifth grid electrode 11. The third and fifth grid electrodes 9 and 11 are arranged to overlap the reduced diameter portions of the fourth grid electrode 10 and these three grid electrodes are supported by a plurality of insulating supporting rods 12 (only two are shown) mounted on thin outer peripheries. These three grid electrodes constitute an electrostatic electron lens structure for focusing the electron beam from the three electrode electron gun structure and to cause the focused electron beam to impinge upon the target 2. A mesh electrode 13 is interposed between the fifth grid electrode 11 and the target 2.
In an image pickup tube constructed as above described there are problems in the method of securing the third, fourth and fifth grid electrodes 9, 10 and 11. More particularly, it is difficult to coaxially arrange at high accuracies the third, fourth and fifth grid electrodes 9, 10 and 11 having constructions described above. Thus, since the third and fifth grid electrodes 9 and 11 are arranged on both sides of the fourth grid electrode 10 having reduced diameter portions on both ends. For this reason, it is necessary to divide into two sections the mandrel adapted to support the three grid electrodes in the assembled condition. For example, a first cylindrical mandrel, not shown, having a diameter substantially equal to the reduced diameter portions on both ends of the fourth grid electrode 10 is inserted into the three grid electrodes and then second cylindrical spacers, not shown, having outer diameters substantially equal to the inner diameters of the third and fifth grid electrodes 9 and 11, respectively, are inserted between the first mandrel and the third grid electrode 9 and between the first mandrel and the fifth grid electrode 11, respectively, thereby holding the three grid electrodes in the assembled condition. Thereafter, a plurality of insulating supporting rods 12 are disposed on the outer surfaces of respective grid electrodes and the grid electrodes and the supporting rods are clamped together by U shaped straps.
With this method of assembling the grid electrodes as the constructions of the mandrels are complicated it is difficult to assemble the grid electrodes at high accuracies. Moreover, as the mandrels are withdrawn after assembling the grid electrodes are often deformed with the result that the grid electrodes become eccentric with each other or their axes become out of alignment. Further, when the clamping straps are welded to respective grid electrodes, as the mandrels are used as one welding electrode the surface of the mandrels is impaired thus decreasing the accuracy of assembling. The warping of the insulating supporting rods also decreases the accuracy of assembling. In the electrostatically focusing type image pickup tube having a diameter more than one inch the resolution of the image pickup tube is degraded due to the deformation of the component parts or eccentric relation among various electrodes which are generated during the assembling of the electrodes that constitute the electrostatic electron lens structure.
On the other hand, in an image pickup tube having a diameter of less than one inch, as it is necessary to increase as far as possible the diameter of the electrostatic focusing lens more careful consideration should be made for the construction and the method of assembling the electrodes. In this case too, there is a problem of degrading the resolution.
FIG. 2 is a diagrammatic longitudinal sectional view showing one example of a small size prior art electrostatically focusing type image pickup tube having an outer diameter of less than one inch. In FIG. 2 elements corresponding to those shown in FIG. 1 are designated by the same reference numerals. The tube shown in FIG. 2 comprises a cylindrical third grid electrode 14 having a reduced diameter portion on one end thereof facing the cathode electrode and fourth and fifth cylindrical grid electrodes 15 and 16 each having the same inner diameter as that of the other end of the third grid electrode 14 facing the target 2. These three grid electrodes are coaxially arranged by means of a plurality of insulating supporting rods 12 to constitute an electrostatic electron lens structure. The image pickup tube shown in FIG. 2 is different from that shown in FIG. 1 in that the fourth grid electrode 15 is not provided with reduced diameter portions on the opposite ends thereof.
The method of assembling the third, fourth and fifth grid electrodes will now be described. Thus, a mandrel 17 having an outer diameter substantially equal to those of the respective grid electrodes is inserted through these grid electrodes after arranging them coaxially. The grid electrodes are spaced from each other by predetermined spacings by using spacers or the like. Thereafter, insulating supporting rods are placed on the outer periphery of respective grid electrodes and then clamping straps 18 embracing the insulating supporting rods 12 are welded to respective grid electrodes. Similar to the image pickup tube shown in FIG. 1, in the case shown in FIG. 2 too there are many problems including the damage of the surface of the mandrel, warping of the insulating supporting rods, and deformation of the ends of respective grid electrodes which are caused by the heat of welding, and the deformation of respective grid electrodes caused by the removal of the mandrel, whereby it is difficult to sufficiently improve the accuracy of assembling.
Comparing the electrode construction shown in FIG. 1 wherein the ends of respective electrodes are overlapped with the electrode construction shown in FIG. 2 wherein all grid electrodes have the same inner diameter, the latter electrode construction is advantageous in that it is possible to assemble the electrodes by using a single mandrel thereby preventing the decrease in the accuracy caused by the use of mandrels having complicated construction. However, as three grid electrodes are arranged coaxially with thin ends faced with each other the cylindrical members that constitute the third, fourth and fifth grid electrodes should be true circles. In the image pickup tubes having constructions as above described departure of the periphery of the electrodes from a true circle or the deformation of the ends of the grid electrodes greatly degrades the resolution characteristics of the image pickup tube as shown by FIG. 4 in which the abscissa represents the amount of deformation of the grid electrode and the ordinate the ratio of deformation of the grid electrode where 100% represents a condition of no deformation.
in the case of the electrode construction shown in FIG. 1 the effect of the departure from the true circle and deformation upon the resolution can be aleviated by the superposed arrangement of the ends of the third, fourth and fifth grid electrods 9, 10 and 11. However the diameter of the focusing lens is smaller than that of the latter electrode construction by about 15% to 20%. For example, where the former electrode construction is adopted for an image pickup tube having an outer diameter of 2/3 inch it is necessary to set the inner diameter of the third and fifth grid electrodes to be about 12 mm, and to set the inner diameter of the reduced diameter portions on both ends of the fourth grid electrode to be from 9.5 to 10.5 mm. By a calculation it was found that the diameter of the electron beam on the imaginary surface of the focusing lens is about 2 mm where OV is applied to the first grid electrode and where the cathode current is maximum. In case of typical operating condition, the diameter of the electron beam is about one half of this value, that is from 1.0 to 1.2 mm. But if the diameter of the electron lens were from 9.5 to 10.5 mm, the diameter of the electron beam would be about 10% of the lens diameter. This means that focusing action is liable to be affected by the spherical aberration of the focusing lens thus greatly increasing the diameter of the electron beam impinging upon the target surface. For this reason, whether the assembling accuracy is high or low the resolution of the image pickup tube decreases as the diameter of the focusing lens decreases. FIG. 5 is a plot showing the variation of the resolution as the inner diameter of the fourth grid electrode is varied from 12 mm to 9.5 mm while maintaining the inner diameters of the third and fifth grid electrodes at a constant value of 12 mm, in which the abscissa represents the inner diameter of the fourth grid electrode whereas the ordinate represents the ratio of resolution. As can be noted from FIG. 5, the resolution decreases with the decrease in the inner diameter of the fourth grid electrode. When the inner diameter decreases below 10 mm, the resolution rapidly decreases due to the influence of the spherical aberration and the deformation of the grid electrode. Although decreasing the electron beam diameter on the plane of the focusing lens is effective to prevent the degradation of the resolution caused by the decrease in the diameter of the focusing lens it is necessary to decrease the diameter of the perforation of the apertured electrode provided for the second grid electrode to about 20.mu. from a normal value of from 20.mu. to 40.mu.. However when the throttling rate is increased in this manner the current density of the electron beam drawn from the cathode electrode will be increased by a factor of two or more, thereby greatly decreasing the so-called emission life of the image pickup tube.